Sputtering target and oxide semiconductor film

ABSTRACT

A sputtering target containing oxides of indium (In), gallium (Ga) and zinc (Zn), which includes a compound shown by ZnGa 2 O 4  and a compound shown by InGaZnO 4 .

TECHNICAL FIELD

The invention relates to a sputtering target and an oxide semiconductorfilm.

BACKGROUND ART

An oxide semiconductor film comprising a metal complex oxide has a highdegree of mobility and visible light transmission, and is used as aswitching device, a driving circuit device or the like of a liquidcrystal display, a thin film electroluminescence display, anelectrophoresis display, a moving particle display or the like.

Examples of an oxide semiconductor film comprising a metal complex oxideinclude an oxide semiconductor film comprising an oxide of In, Ga and Zn(IGZO). An oxide semiconductor film obtained by using an IGZO sputteringtarget has an advantage that it has a higher mobility than that of anamorphous silicon film, and hence has been attracting attention (seePatent Documents 1 to 10).

An IGZO target is known to be composed mainly of a compound shown byInGaO₃(ZnO)_(m) (wherein m is an integer of 1 to 20). However, ifsputtering (DC sputtering, for example) is conducted by using an IGZOsputtering target, a problem arises that the compound shown byInGaO₃(ZnO)_(m) grows abnormally to cause abnormal discharge, leading tothe formation of a defective film.

Furthermore, an IGZO sputtering target is produced by mixing rawmaterial powder to form a mixture, prefiring, pulverizing, granulatingand molding the mixture to form a molded product, and sintering andreducing the molded product. However, due to a large number of steps,this process has a disadvantage that the productivity of a sputteringtarget is lowered, resulting in an increased production cost.

Therefore, it is preferable to omit even one of these steps. However, noimprovement has heretofore been made on the production process, and anIGZO target is being produced by a conventional process.

In addition, the sputtering target obtained by conventional processeshas a conductivity of about 90 S/cm (specific bulk resistance: 0.011Ωcm); in other words, has a high resistance. It is difficult to obtainby conventional processes a target which does not suffer from crackingor other problems during sputtering.

Compounds, which are contained in an IGZO target and shown byInGaO₃(ZnO)₂, InGaO₃(ZnO)₃, InGaO₃(ZnO)₄, InGaO₃(ZnO)₅ and InGaO₃(Zn0)₇,as well as the production method thereof are disclosed in PatentDocuments 11 to 15.

However, compounds shown by ZnGa₂O₄ and InGaZnO₄ are not obtained inPatent Documents 11 to 15. As for the particle size of the raw materialpowder used in Patent Documents 11 to 15, these patent documents onlystate that a particularly preferred particle size is 10 μm or less.Furthermore, although these patent documents state that these compoundscan be used in a semiconductor device, no statement is made on thespecific resistance value thereof and the use thereof in a sputteringtarget.

Patent Document 1: JP-A-H8-295514

Patent Document 2: JP-A-H8-330103

Patent Document 3: JP-A-2000-044236

Patent Document 4: JP-A-2006-165527

Patent Document 5: JP-A-2006-165528

Patent Document 6: JP-A-2006-165529

Patent Document 7: JP-A-2006-165530

Patent Document 8: JP-A-2006-165531

Patent Document 9: JP-A-2006-165532

Patent Document 10: JP-A-2006-173580

Patent Document 11: JP-A-S63-239117

Patent Document 12: JP-A-S63-210022

Patent Document 13: JP-A-S63-210023

Patent Document 14: JP-A-S63-210024

Patent Document 15: JP-A-S63-265818

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a sputtering target which cansuppress abnormal discharge which occurs during formation of an oxidesemiconductor film by the sputtering method, and is capable of formingan oxide semiconductor film which is improved in surface smoothnesswithout film quality disorders.

Another object of the invention is to provide a sputtering target whichmaintains the properties of an IGZO sputtering target, has a low bulkresistance, a high density, and more uniform-fine-particle-sizestructure, as well as a high degree of transverse rupture strength.

Still another object of the invention is to provide a sputtering targetwhich can suppress occurrence of abnormal discharge even though it is anIGZO sputtering target and is used in DC sputtering.

Further another object of the invention is to provide a method forproducing a sputtering target which can shorten the production processwithout impairing the properties of an IGZO sputtering target.

The inventors have found that, in an IGZO sputtering target whichcontains an oxide of indium (In), gallium (Ga) and zinc (Zn), a compoundshown by ZnGa₂O₄ inhibits abnormal growth of a compound shown byInGaO₃(ZnO)_(m) (wherein m is an integer of 2 to 20), whereby abnormaldischarge during sputtering can be suppressed, and a compound shown byInZnGaO₄ inhibits abnormal growth of a compound shown by InGaO₃(ZnO)_(m)(wherein m is an integer of 2 to 20), whereby abnormal discharge duringsputtering can be suppressed.

In addition, the inventors have found that, the bulk resistance of asputtering target itself can be reduced by adding a metal element withan atomic valency of positive tetravalency or higher, whereby abnormaldischarge can be suppressed (First Invention).

Furthermore, the inventors have found that, occurrence of abnormaldischarge during sputtering can be suppressed by adding to an IGZOsputtering target which comprises InGaZnO₄ as a main component a metalelement with an atomic valency of positive tetravalency or higher(Second Invention).

In addition, the inventors have found that, in the method for producingan IGZO sputtering target, the prefiring step and the reduction step canbe omitted by mixing/pulverizing, according to a specificmixing/pulverizing method, indium oxide, gallium oxide and zinc oxide,or raw material powder containing them as main components, thereby toadjust the specific surface area or the median diameter of the rawmaterial mixed powder and the powder after pulverization (ThirdInvention).

The invention provides the following sputtering target or the like.

First Invention First Embodiment

1. A sputtering target containing oxides of indium (In), gallium (Ga)and zinc (Zn), which comprises a compound shown by ZnGa₂O₄ and acompound shown by InGaZnO₄.2. The sputtering target according to 1, wherein an atomic ratio shownby In/(In+Ga+Zn), an atomic ratio shown by Ga/(In+Ga+Zn) and an atomicratio shown by Zn/(In+Ga+Zn) satisfy the following relationship:

0.2<In/(In+Ga+Zn)<0.77

0.2<Ga/(In+Ga+Zn)<0.50

0.03<Zn/(In+Ga+Zn)<0.50.

3. The sputtering target according to 1 or 2, wherein the atomic ratioshown by In/(In+Ga+Zn) and the atomic ratio shown by Ga/(In+Ga+Zn)satisfy the following relationship:

In/(In+Ga+Zn)>Ga/(In+Ga+Zn).

4. The sputtering target according to any one of 1 to 3, wherein thesputtering target comprises a metal element with an atomic valency ofpositive tetravalency or higher, and the content of the metal elementwith an atomic valency of positive tetravalency or higher relative tothe total metal elements [metal element with an atomic valency ofpositive tetravalency or higher/total metal elements:atomic ratio] is0.0001 to 0.2.5. The sputtering target according to 4, wherein the metal element withan atomic valency of positive tetravalency or higher is one or moreelements selected from tin, zirconium, germanium, cerium, niobium,tantalum, molybdenum and tungsten.6. The sputtering target according to any one of 1 to 5, which has abulk resistance of less than 5×10⁻³ Ωcm.7. An oxide semiconductor film which is obtained by film formation bysputtering using the sputtering target according to any one of 1 to 6.

First Invention Second Embodiment

1. A sputtering target containing oxides of indium (In), gallium (Ga)and zinc (Zn), which comprises a homologous structure compound shown byInGaO₃(ZnO)_(m) (wherein m is an integer of 1 to 20) and a spinelstructure compound shown by ZnGa₂O₄.2. The sputtering target according to 1, which comprises at least ahomologous structure compound shown by InGaZO₄.3. The sputtering target according to 1 or 2, wherein the spinelstructure compound shown by ZnGa₂O₄ has an average particle diameter of10 μm or less.4. The sputtering target according to any one of 1 to 3, which has asintered body density of 6.0 g/cm³ or more.5. The sputtering target according to any one of 1 to 4, which has asurface roughness (Ra) of 2 μm or less and an average transverse rupturestrength of 50 MPa or more.6. The sputtering target according to any one of 1 to 5, wherein thecontent of each of Fe, Al, Si, Ni and Cu is 10 ppm (weight) or less.7. A method of producing a sputtering target according to any one of 1to 6 comprising the steps of:

pulverizing and mixing/granulating indium oxide, gallium oxide and zincoxide to prepare a mixture;

molding the mixture to obtain a molded product; and

sintering the molded product in an oxygen stream or under an oxygenpressure at a temperature of 1250° C. or higher and lower than 1450° C.

8. An oxide semiconductor film which is obtained by film formation bysputtering using the sputtering target according to any one of 1 to 6.

Second Invention

1. A sputtering target comprising a compound shown by InGaZnO₄ as a maincomponent, which further contains a metal element with an atomic valencyof positive tetravalency or higher.2. The sputtering target according to 1, wherein the content of themetal element with an atomic valency of positive tetravalency or higheris 100 ppm to 10000 ppm relative to the total metal elements in thesputtering target.3. The sputtering target according to 1, wherein the content of themetal element with an atomic valency of positive tetravalency or higheris 200 ppm to 5000 ppm relative to the total metal elements in thesputtering target.4. The sputtering target according to 1, wherein the content of themetal element with an atomic valency of positive tetravalency or higheris 500 ppm to 2000 ppm relative to the total metal elements in thesputtering target.5. The sputtering target according to any one of 1 to 4, which has abulk resistance of less than 1×10⁻³ Ωcm.6. The sputtering target according to any one of 1 to 5, wherein themetal element with an atomic valency of positive tetravalency or higheris at least one element selected from the group consisting of tin,zirconium, germanium, cerium, niobium, tantalum, molybdenum andtungsten.

Third Invention

1. A method for producing a sputtering target comprising the steps of:

preparing, as raw material powder, mixed powder containing indium oxidepowder having a specific surface area of 6 to 10 m²/g, gallium oxidepowder having a specific surface area of 5 to 10 m²/g and zinc oxidepowder having a surface area of 2 to 4 m²/g, the specific surface areaof the entire mixed powder being 5 to 8 m²/g;

mixing/pulverizing the raw material powder by means of a wet mediumstirring mill to increase the specific surface area of the entire mixedpowder by 1.0 to 3.0 m²/g;

molding the raw material powder to obtain a molded product; and

sintering the molded product in an oxygen atmosphere at a temperature of1250 to 1450° C.

2. A method for producing a sputtering target comprising the steps of:

preparing, as raw material powder, mixed powder containing indium oxidepowder having a median diameter of particle size distribution of 1 to 2μm, gallium oxide powder having a median diameter of particle sizedistribution of 1 to 2 μm and zinc oxide powder having a median diameterof particle size distribution of 0.8 to 1.6 μm, the median diameter ofparticle size distribution of the entire mixed powder being 1.0 to 1.9μm;

mixing/pulverizing the raw material powder by means of a wet mediumstirring mill to allow the raw material powder to have a median diameterof 0.6 to 1 μm;

molding the raw material powder to obtain a molded product; and

sintering the molded product in an oxygen atmosphere at a temperature of1250 to 1450° C.

3. The method for producing a sputtering target according to 1 or 2,wherein the sintering is performed without conducting prefiring.4. The method for producing a sputtering target according to any one of1 to 3, wherein the density of a sintered body obtained in the sinteringstep is 6.0 g/cm³ or more.

According to the invention, it is possible to provide a sputteringtarget which can suppress abnormal discharge which occurs when an oxidesemiconductor film is formed by the sputtering method and is capable offorming an oxide semiconductor film which is free from film qualitydisorders and has improved surface smoothness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray chart of a target prepared in Example 1;

FIG. 2 is an X-ray chart of a target prepared in Example 2;

FIG. 3 is an X-ray chart of a target prepared in Example 3;

FIG. 4 is an X-ray chart of a target prepared in Comparative Example 1;

FIG. 5 is a graph showing the relationship between the amount of a metalwith an atomic valency of positive tetravalency or higher and the bulkresistance of a sintered body;

FIG. 6 is an X-ray diffraction chart of a target prepared by adding tin;

FIG. 7 is an X-ray diffraction chart of a sintered body prepared inExample 8;

FIG. 8 is a graph showing the relationship between the amount of a tinelement and the bulk resistance of a sintered body, and

FIG. 9 is a graph showing the relationship between the amount of a metalelement with an atomic valency of positive tetravalency or higher andthe bulk resistance of a sintered body.

BEST MODE FOR CARRYING OUT THE INVENTION First Invention FirstEmbodiment

The sputtering target of the invention (hereinafter often referred to as“the target of the invention”) contains oxides of indium (In), gallium(Ga) and zinc (Zn), and comprises a compound shown by ZnGa₂O₄ and acompound shown by InGaZnO₄.

By allowing a compound shown by ZnGa₂O₄ and a compound shown by InGaZnO₄to be generated in a sputtering target, abnormal growth of a compoundshown by InGaO₃(ZnO)_(m) (wherein m is an integer of 2 to 20) can besuppressed, and abnormal discharge of the sputtering during sputteringcan be suppressed. In addition, since the crystal particle size can bereduced, oxygen deficiency is generated in the crystal interface,whereby bulk resistance can be reduced.

In addition, the sputtering target of the invention contains a pluralityof crystal systems such as a compound shown by InGaO₃(ZnO)_(m) (whereinm is an integer of 2 to 20) and a compound shown by ZnGa₂O₄, oxygendeficiency occurs due to the incommensuration of crystals in the crystalgrain boundary, and as a result, carriers are generated in the target.These carriers lower the resistance of the target, whereby abnormaldischarge during sputtering can be suppressed.

In the sputtering target of the invention, it is preferred that anatomic ratio shown by In/(In+Ga+Zn), an atomic ratio shown byGa/(In+Ga+Zn) and an atomic ratio shown by Zn/(In+Ga+Zn) satisfy thefollowing relationship:

0.2<In/(In+Ga+Zn)<0.77

0.2<Ga/(In+Ga+Zn)<0.50

0.03<Zn/(In+Ga+Zn)<0.50.

The above-mentioned atomic ratios are obtained by adjusting the mixingratio of the indium compound, the gallium compound and the zinc compoundbefore sintering, which will be mentioned later.

If the In/(In+Ga+Zn) is 0.77 or more, the conductivity of the resultingoxide semiconductor film may be increased, making it difficult to beused as a semiconductor. If the In/(In+Ga+Zn) is 0.2 or less, the oxidesemiconductor film obtained by film formation may have a lowered carriermobility when used as a semiconductor.

If the Ga/(In+Ga+Zn) is 0.5 or more, when used as a semiconductor, thecarrier mobility of the oxide semiconductor film obtained by filmformation may be lowered. On the other hand, if the Ga/(In+Ga+Zn) is 0.2or less, the conductivity of the oxide semiconductor film obtained byfilm formation may be increased, making it difficult to be used as asemiconductor. In addition, the semiconductor properties may be variedor the threshold voltage (Vth) shift may be increased due to disturbancesuch as heating.

If the Zn/(In+Ga+Zn) is 0.03 or less, the oxide semiconductor film maybe crystallized. On the other hand, if the Zn/(In+Ga+Zn) is 0.5 or more,the oxide semiconductor film itself may not have sufficient stability,resulting in an increased Vth shift.

The atomic ratio of each element in the above-mentioned target can beobtained by measuring the amount of each element by ICP (InductivelyCoupled Plasma).

In the sputtering target of the invention, the atomic ratio shown byIn/(In+Ga+Zn) and the atomic ratio shown by Ga/(In+Ga+Zn) preferablysatisfy the following formula:

In/(In+Ga+Zn)>Ga/(In+Ga+Zn).

The sputtering target satisfying this formula is capable of producing anoxide semiconductor film which has a high degree of stability, thesuppressed Vth shift, and long-term stability.

The sputtering target of the invention preferably contains a metalelement with an atomic valency of positive tetravalency or higher, andthe content of the metal element with an atomic valency of positivetetravalency or higher relative to the total metal elements [metalelement with an atomic valency of positive tetravalency or higher/totalmetal elements:atomic ratio] is 0.0001 to 0.2.

Since the sputtering target contains a metal element with an atomicvalency of positive tetravalency of higher, the target itself has areduced bulk resistance, whereby occurrence of abnormal discharge duringsputtering by the target can be suppressed.

If the content ratio [metal element with an atomic valency of positivetetravalency or higher/total metal elements:atomic ratio] is less than0.0001, the effects of decreasing the bulk resistance may be small. Onthe other hand, if the content ratio [metal element with an atomicvalency of positive tetravalency or higher/total metal elements:atomicratio] exceeds 0.2, the stability of the oxide semiconductor film may belowered.

Examples of preferred metal element with an atomic valency of positivetetravalency or higher include tin, zirconium, germanium, cerium,niobium, tantalum molybdenum and tungsten. Of these, tin, cerium andzirconium are more preferable.

The above-mentioned metal element with an atomic valency of positivetetravalency or higher is added, for example, as a metal oxide, to theraw materials of the sputtering target such that the content of metalelement falls within the above-mentioned range.

In addition to the above-mentioned metal element with an atomic valencyof positive tetravalency or higher, the sputtering target of theinvention may contain, for example, hafnium, rhenium, titanium, vanadiumor the like within an amount range which does not impair theadvantageous effects of the invention.

Second Embodiment

The sputtering target of the invention (hereinafter often referred to asthe target of the invention) contains oxides of indium (In), gallium(Ga) and zinc (Zn), and comprises a homologous compound shown byInGaO₃(ZnO)_(m) (m is an integer of 1 to 20) and a spinel structurecompound shown by ZnGa₂O₄.

The homologous compound is a compound having a homologous phase.

The homologous phase (Homologous Series) is, for example, a magneliphase shown by a compositional formula Ti_(n)O_(2n-1), taking n as anatural number. In such a phase, there are a series of compounds inwhich n varies continuously.

Specific examples of the homologous compound include In₂O₃(ZnO)_(m)(wherein m is an integer of 2 to 20), InGaO₃(ZnO)_(m) (wherein m is aninteger of 2 to 20), or the like.

As described in “Crystal Chemistry” (Mitsuoki Nakahira, Kodansha, 1973)or the like, an AB₂X₄ type or an A₂BX₄ type is called a spinelstructure, and a compound having such a crystal structure is generallycalled a spinel structure compound.

In a common spinel structure, anions (usually oxygen) are filled bycubic closest packing with cations being present in part of tetrahedronor octahedron clearances.

A substituted-type solid solution in which some of the atoms and ions inthe crystal structure are replaced with other atoms and an interstitialsolid solution in which other atoms are added to the sites betweengratings are also included in the spinel structure compounds.

The crystal conditions of the compound in the target can be judged byobserving a sample extracted from the target (sintered body) by theX-ray diffraction method.

The spinel structure compound constituting the target of the inventionis a compound shown by ZnGa₂O₄. That is, in the X-ray diffraction, thecompound shows a peak pattern of No. 38-1240 or an analogous (shifted)pattern of the Joint Committee on Powder Diffraction Standards (JCPDS)database.

By allowing a compound shown by ZnGa₂O₄. to be formed in the sputteringtarget, abnormal growth of a compound shown by InGaO₃(ZnO)_(m) (whereinm is an integer of 2 to 20) can be suppressed, whereby abnormaldischarge of a target during sputtering can be suppressed. Preferably,by allowing a compound shown by InGaZnO₄ to be formed, abnormal growthof a compound shown by InGaO₃(ZnO)_(m) (wherein m is an integer of 2 to20) can be further suppressed.

By suppressing abnormal growth of a compound shown by InGaO₃(ZnO)_(m)(wherein m is an integer of 2 to 20), it is possible to increase thetransverse rupture strength of the target, whereby cracking of thetarget during sputtering can be suppressed.

Since the sputtering target of the invention contains a plurality ofcrystal systems of the homologous structure compound shown byInGaO₃(ZnO)_(m) (wherein m is an integer of 1 to 20) and the spinelstructure compound shown by ZnGa₂O₄, oxygen deficiency occurs due to theincommensuration of the crystals in the crystal grain boundary, and as aresult, carriers are generated in the target. These carriers lower theresistance of the target, whereby abnormal discharge during sputteringcan be suppressed.

In the target of the invention, the average particle size of the spinelstructure compound shown by ZnGa₂O₄ is preferably 10 μm or less, morepreferably 5 μm or less.

By allowing the average particle size of the spinel structure compoundshown by ZnGa₂O₄ to be 10 μm or less, the particle growth of thecompound shown by InGaO₃(ZnO)_(m) (wherein m is an integer of 2 to 20)can be more surely suppressed, and the transverse rupture strength ofthe target can be increased, whereby cracking of the target duringsputtering is suppressed.

The average particle size of the above-mentioned spinel structurecompound shown by ZnGa₂O₄ can be evaluated by observing a secondaryelectron image of a scanning electron microscope (SEM), for example.

The sputtering target of the above-mentioned first and secondembodiments preferably has a bulk resistance of less than 5×10⁻³ Ωcm,more preferably less than 2×10⁻³ Ωcm. If the bulk resistance is 5×10⁻³Ωcm or more, abnormal discharge may be induced during sputtering andforeign matters (nodules) may be generated.

The bulk resistance of the target of the invention can be measured bythe four probe method.

The sputtering target of the invention preferably has a sintered bodydensity of 6.0 g/cm³ or more.

By allowing the density of a sintered body of the target to be 6.0 g/cm³or more, the transverse rupture strength of the target can be increased,thereby suppressing cracking of the target during sputtering. On theother hand, if the density of a sintered body of the target is less than6.0 g/cm³, the target surface may be blackened to cause abnormaldischarge.

In order to obtain a high-density sintered body, it is preferable toperform molding by the cold isostatic press (CIP) method, the hotisostatic press (HIP) method, or the like.

It is preferred that the target of the invention have a surfaceroughness (Ra) of 2 μm or less and an average transverse rupturestrength of 50 MPa or more. More preferably, the target of the inventionhas a surface roughness (Ra) of 0.5 μm or less and an average transverserupture strength of 55 MPa or more.

By allowing the surface roughness (Ra) of the target to be 2 μm or less,the average transverse rupture strength of the target can be maintained50 MPa or more, whereby cracking of the target during sputtering can besuppressed.

The surface roughness (Ra) can be measured by the AFM method and theaverage rupture transverse strength can be measured according to JIS R1601.

It is preferred that the target of the invention contain Fe, Al, Si, Niand Cu in an amount of 10 ppm (weight) or less.

Fe, Al, Si, Ni and Cu are the impurities of the target of the invention.By allowing the content of each of Fe, Al, Si, Ni and Cu to be 10 ppm(weight) or less, variation in the threshold voltage of an oxidesemiconductor film obtained by film formation using this target can besuppressed, whereby stable operation conditions can be obtained.

The content of the above-mentioned impurity elements can be measured bythe inductively coupled plasma (ICP) spectrophotometry.

In addition to oxides of indium (In), gallium (Ga) and zinc (Zn), thesputtering target of the invention may contain a metal element with anatomic valency of positive tetravalency within an amount range whichdoes not impair the advantageous effects of the invention.

An oxide semiconductor film obtained by using the sputtering target ofthe invention is amorphous, and exhibits stable semiconductor propertieswithout the effects of carrier generation (doping effects) even though ametal element with an atomic valency of positive tetravaleny iscontained.

—Method for Producing a Sputtering Target

The sputtering target of the above-mentioned first and secondembodiments can be produced, for example, by pulverizing andmixing/granulating indium oxide, gallium oxide and zinc oxide to preparea mixture, molding the mixture to obtain a molded product, andsubjecting the molded product to a heat treatment in an oxygen stream orunder an oxygen pressure at a temperature of 1250° C. or higher andlower than 1450° C.

The raw materials of the sputtering target of the invention are indiumoxide, gallium oxide and zinc oxide. Preferably, the raw materials ofthe sputtering target are indium oxide powder with a specific area of 6to 10 m²/g, gallium oxide powder with a specific area of 5 to 10 m²/gand zinc oxide powder with a specific area of 2 to 4 m²/g, or indiumoxide powder with a median diameter of 1 to 2 μm, gallium oxide powderwith a median diameter of 1 to 2 μm and zinc oxide powder with a mediandiameter of 0.8 to 1.6 μm.

The purity of each of the above-mentioned raw materials is normally 2N(99 mass %) or more, preferably 3N (99.9 mass %) or more, and morepreferably 4N (99.99 mass %) or more. If the purity is lower than 2N, alarge amount of impurities such as Fe, Al, Si, Ni and Cu may becontained. Due to the presence of these impurities, troubles occur thatthe operation of an oxide semiconductor film prepared by using thistarget becomes unstable or the like.

A common pulverizer may be used for pulverizing the above-mentioned rawmaterials. For example, the raw materials can be uniformly mixed andpulverized by means of a wet medium stirring mill, a beads mill, or anultrasonic apparatus.

The raw materials are weighed such that the mixing ratio In:Ga:Znbecomes 45:30:25 in weight ratio (In:Ga:Zn=1:1:1 in molar ratio) orIn₂O₃:Ga₂O₃:ZnO becomes 51:34:15 in weight ratio (In₂O₃:Ga₂O₃:ZnO=1:1:1in molar ratio).

The raw materials are mixed such that, in the sputtering target of theinvention, an atomic ratio shown by In/(In+Ga+Zn), an atomic ratio shownby Ga/(In+Ga+Zn) and an atomic ratio shown by Zn/(In+Ga+Zn) satisfy thefollowing formulas, for example.

0.2<In/(In+Ga+Zn)<0.77

0.2<Ga/(In+Ga+Zn)<0.50

0.03<Zn/(In+Ga+Zn)<0.50

The mixed powder is prepared such that the specific surface area of eachof the raw materials is increased by 1.0 to 2.5 m²/g or such that theaverage median diameter of each of the raw materials becomes 0.6 to 1.0μm after pulverization and mixing/granulation.

If the specific surface area of each of the raw materials is increasedby less than 1.0 m²/g or the average median diameter of each of the rawmaterials is increased by less than 0.6 μm, the amount of impuritieswhich are admixed from a pulverizer or the like during pulverization andmixing/granulation may be increased.

In the above-mentioned pulverization and mixing/granulation, if thespecific surface areas of indium oxide and gallium oxide beforepulverization and mixing/granulation are almost the same, pulverizationand mixing/granulation can be performed more effectively. It ispreferred that the difference in specific surface area between indiumoxide and gallium oxide before pulverization and mixing/granulation be 3m²/g or less. If the difference in specific surface area is outside theabove-mentioned range, pulverization and mixing/granulation cannot beperformed efficiently. As a result, gallium oxide particles may remainin the resulting sintered body.

If the median diameter of indium oxide and the median diameter ofgallium oxide before pulverization and mixing/granulation are almost thesame, pulverization and mixing/granulation can be performed moreefficiently. It is preferred that the difference in median diameterbetween indium oxide and gallium oxide before pulverization andmixing/granulation be 1 μm or less. If the difference in median diameteris not within this range, gallium oxide particles may remain in theresulting sintered body.

As the molding treatment for molding the above-mentioned mixture, diemolding, cast molding, injection molding, and the like can be given. Inorder to obtain a sintered body having a high degree of density, it ispreferable to mold by CIP (isostatic press) or the like.

For the molding treatment, a molding aid such as polyvinyl alcohol,methyl cellulose, polywax, oleic acid or the like may be used.

It is possible to produce a sintered body for the sputtering target ofthe invention by subjecting a molded product obtained by theabove-mentioned method to firing.

The temperature for firing is 1250° C. or higher and lower than 1450°C., preferably 1300° C. or higher and lower than 1450° C. The firingtime is normally 2 to 100 hours, preferably 4 to 40 hours.

If the firing temperature is lower than 1250° C., a resulting sinteredbody may not have an increased density. If the firing temperature is1450° C. or higher, zinc is evaporated, whereby the composition of thesintered body varies and/or voids are generated in the target.

It is preferred that the above-mentioned firing be performed in anoxygen stream or under an oxygen pressure. By performing firing in anoxygen atmosphere, evaporation of zinc can be suppressed, whereby asintered body having no voids can be produced. InGaZnO₄ and ZnGa₂O₄ areformed in this sintered body. Formation of InGaZnO₄ and ZnGa₂O₄ can beconfirmed by the X-ray diffraction method.

The sputtering target of the invention can be produced by subjecting thesintered body after firing to, for example, polishing to have a desiredsurface roughness.

For example, the above-mentioned sintered body is ground by means of asurface grinder to allow the average surface roughness (Ra) to be 5 μmor less, preferably 2 μm or less. Furthermore, the sputtering surface isthen subjected to mirror finishing, whereby the average surfaceroughness (Ra) can be to 1,000 angstroms or less.

There are no restrictions on the type of mirror finishing (polishing).Known polishing technologies such as mechanical polishing, chemicalpolishing, and mechanochemical polishing (combination of mechanicalpolishing and chemical polishing) can be used. For example, it ispossible to polish to a roughness of #2000 or more by using a fixedabrasive polisher (polishing solution: water), or, after lapping bymeans of a free abrasive lap (abrasive: SiC paste or the like), it ispossible to lap by using diamond paste instead of the abrasive.

In the method for producing the sputtering target of the invention, itis preferred that the resulting sputtering target be subjected to acleaning treatment.

Examples of the cleaning treatment include air blowing and washing withrunning water. If cleaning (removal of foreign matters) is performed byair blowing, foreign matters can be effectively removed by absorbing theair by means of a dust collector facing the nozzle.

After the above-mentioned cleaning treatments such as air blowing andwashing with running water, it is preferred that ultrasonic cleaning orthe like be further conducted. It is effective to conduct thisultrasonic cleaning by generating multiple oscillation within afrequency of 25 to 300 KHz. For example, ultrasonic cleaning may beperformed by generating multiple oscillation of 12 kinds of frequenciesof from 25 to 300 KHz every 25 KHz.

After bonding to a backing plate, the sputtering target can be installedin a sputtering apparatus.

According to the method for producing the sputtering target of theinvention, it is possible to obtain a high-density sintered body for asputtering target without the need of a prefiring step. In addition, itis possible to obtain a sintered body with a low bulk resistance withthe need of a reduction step. The sputtering target of the invention canbe produced with a high degree of productivity since the productionthereof does not require the above-mentioned prefiring and reductionsteps.

An oxide semiconductor film can be formed by using the target of theinvention. As the film forming method, the RF magnetron sputteringmethod, the DC magnetron sputtering method, the electron beam depositionmethod, the ion plating method or the like can be used. Of these, the RFmagnetron sputtering method can preferably be used. If the bulkresistance of the target exceeds 1 Ωcm, a stable sputtering state can bemaintained without causing abnormal discharge if the RF magnetronsputtering method is used. If the bulk resistance of the target is 10mΩcm or less, the DC magnetron sputtering method, which is industriallyadvantageous, can also be used.

As a result, a stable sputtering state can be maintained withoutsuffering from abnormal discharge, and stable film formation can beperformed continuously on the industrial scale.

An oxide semiconductor film formed by using the sputtering target of theinvention generates only a small amount of nodules or particles due tothe high density of the sputtering target, and hence, is improved infilm properties (excellent surface smoothness with no film qualitydisorders).

Second Invention

The sputtering target of the invention is an IGZO sputtering targetwhich is produced mainly from indium oxide, gallium oxide and zincoxide, comprises a compound shown by InGaZnO₄ as a main component andfurther contains a metal element with an atomic valency of positivetetravalency or higher. In an IGZO sputtering target to which a metalelement with an atomic valency of positive tetravalency or higher isadded, the bulk resistance of the target itself can be reduced and theoccurrence of abnormal discharge during DC sputtering can be suppressed.

Furthermore, in the production process of the target, it is possible toomit a reduction step for reducing the bulk resistance of the target.Therefore, productivity can be improved with a reduced production cost.

The presence of a compound (crystal) shown by InGaZnO₄ as a maincomponent means that a structure other than InGaZnO₄ cannot be confirmedby the X-ray diffraction analysis, or means that, even though astructure other than InGaZnO₄ is confirmed, the intensity thereof issmaller than that of InGaZnO₄.

In the sputtering target of the invention, the amount (weight) of themetal element with an atomic valency of positive tetravalency or higherrelative to the total metal elements in the target is preferably 100 ppmto 10000 ppm. If the amount is less than 100 ppm, the effects of addinga metal element with an atomic valency of positive tetravalency orhigher may be small, and if the amount exceeds 10000 ppm, an oxide thinfilm may have an insufficient stability or may have a lowered carriermobility. The amount of a metal element with an atomic valency ofpositive tetravalency or higher is preferably 200 ppm to 5000 ppm, morepreferably 500 ppm to 2000 ppm.

In the sputtering target of the invention, it is preferred that the bulkresistance of the target be less than 1×10⁻³ Ωcm. If the bulk resistanceis 1×10⁻³ Ωcm or more, when DC sputtering is conducted for a long periodof time, spark is generated due to abnormal discharge to cause thetarget to be cracked or the properties of the resulting film as an oxidesemiconductor film may be lowered due to the adhesion of] particleswhich have jumped out from the target by spark to a formed film on asubstrate.

The bulk resistance is a value which is measured by means of aresistivity meter according to the four probe method.

The sputtering target of the invention can be produced by mixing allpowder of indium oxide, gallium oxide, zinc oxide and a materialcontaining a metal element with an atomic valency of positivetetravalency or higher to form a mixture and pulverizing and sinteringthe mixture.

As the raw material containing a metal element with an atomic valency ofpositive tetravalency or higher, a simple metal element or a metal oxidecan be used. As the metal element with an atomic valency of positivetetravalency or higher, one or a plurality of metal elements may beappropriately selected from tin, zirconium, germanium, cerium, niobium,tantalum, molybdenum and tungsten.

As for the raw material powder, it is preferred that the specificsurface area of the indium oxide powder be 8 to 10 m²/g, the specificsurface area of the gallium oxide powder be 5 to 10 m²/g, and thespecific surface area of the zinc oxide powder be 2 to 4 m²/g. It ispreferred that the median diameter of the indium oxide powder be 1 to 2μm, the median diameter of the gallium oxide powder be 1 to 2 μm and themedian diameter of the zinc oxide powder be 0.8 to 1.6 μm.

Furthermore, it is preferable to use powder in which the specificsurface area of the indium oxide powder and the specific surface area ofthe gallium oxide powder are almost the same. By using such powder,pulverization and mixing can be conducted more efficiently.Specifically, it is preferable to make a specific surface areadifference 3 m²/g or less. If the specific surface area is significantlydifferent between the indium oxide powder and the gallium oxide powder,pulverization and mixing may not be conducted efficiently, and as aresult, gallium oxide particles may remain in the resulting sinteredbody.

In the raw material powder, the indium oxide powder, the gallium oxidepowder and the zinc oxide powder are mixed such that the mixing ratiothereof (indium oxide powder:gallium oxide powder:zinc oxide powder)becomes about 45:30:25 (In:Ga:Zn=1:1:1 in molar ratio) or about 51:34:15(In₂O₃:Ga₂O₃:ZnO=1:1:1).

It is preferred that the amount of the material containing the metalelement with an atomic valency of positive tetravalency or higher be 100ppm to 10000 ppm relative to the total metal elements in the target. Theamount of the material containing the metal element with an atomicvalency of positive tetravalency or higher is appropriately adjustedbased on the amounts of the indium oxide powder, the gallium oxidepowder and the zinc oxide powder.

As long as the mixed powder containing the indium oxide powder, thegallium oxide powder, the zinc oxide powder and the material containinga metal element with an atomic valency of positive tetravalency orhigher is used, other components for improving the properties of thesputtering target may be added.

The mixed powder is pulverized by means of a wet medium stirring mill,for example. At this time, it is preferred that pulverization beperformed such that the specific area of the mixed powder is increasedby 1.5 to 2.5 m²/g after pulverization, or such that the average mediandiameter becomes 0.6 to 1 μm after pulverization. By using the thusadjusted raw material powder, it is possible to obtain a high-densitysintered body for an IGZO sputtering target without conducting theprefiring step. Also, the reduction step becomes unnecessary.

If an increase in the specific area of the raw material mixed powder isless than 1.0 m²/g or if the average median diameter of the raw materialmixed powder after pulverization exceeds 1 μm, the sintered density maynot be sufficiently large. If an increase in the specific surface areaof the raw material mixed powder exceeds 3.0 m²/g or the average mediandiameter after pulverization is less than 0.6 μm, the amount ofcontaminants (the amount of admixed impurities) from a pulverizer or thelike at the time of pulverization may be increased.

Here, the specific area of each powder is a value measured by the BETmethod. The median diameter of particle distribution of each powder is avalue measured by means of a particle size distribution meter. Thesevalues can be adjusted by pulverizing the powder by a dry pulverizingmethod, a wet pulverizing method or the like.

After drying the raw material after pulverization by means of a spraydryer or the like, the raw material is then molded. Molding can beconducted by a known method such as the pressure molding method and theisostatic press method.

Then, the resulting molded product is sintered to obtain a sinteredbody. It is preferred that sintering be performed at 1500 to 1600° C.for 2 to 20 hours. As a result, a sintered body for an IGZO sputteringtarget with a sintered body density of 6.0 g/cm³ or more can beobtained. If the sintering temperature is lower than 1500° C., densitymay not be improved. If the temperature exceeds 1600° C., zinc may beevaporated, whereby the composition of a sintered body is varied orvoids are generated in a sintered body due to evaporation.

It is preferred that sintering be performed in an oxygen atmosphere bycirculating oxygen or under a pressure. By this sintering method, zincevaporation can be suppressed, whereby a sintered body having no voidscan be obtained.

A sintered body produced by the above-mentioned method has a density ashigh as 6.0 g/cm³ or more. Therefore, only a small amount of nodules orparticles are generated during use, and hence, an oxide semiconductorfilm improved in film properties can be prepared.

In the resulting sintered body, InGaZnO₄ is generated as a maincomponent. Generation of InGaZnO₄ can be confirmed by identifying thecrystal structure by the X-ray diffraction method.

A sputtering target can be obtained by subjecting the resulting sinteredbody to polishing, cleaning or the like as in the case of theabove-mentioned first invention.

By conducting sputtering using a sputtering target after bonding, anIGZO oxide semiconductor film containing oxides of In, Ga and Zn as maincomponents can be formed on an object such as a substrate.

An oxide thin film obtained by using the target of the invention is anamorphous film, and since the metal element with an atomic valency oftetravalency or higher which has been added does not demonstrate dopingeffects (effects of carrier generation), it is fully satisfactory as afilm of which the electron density is reduced. Therefore, when used asan oxide semiconductor film, it exhibits a high degree of stability, andoperates stably as a semiconductor due to a suppressed Vth shift.

Third Invention

In the method for producing the sputtering target of the invention,mixed powder of indium oxide powder, gallium oxide powder and zinc oxidepowder, or powder containing indium oxide, gallium oxide and zinc oxideas main components is used as a raw material. One of the characteristicfeatures of the invention is to use each raw material having aprescribed specific surface area or a specific median diameter ofparticle diameter distribution.

Furthermore, the method of the invention is characterized in that itcomprises the step of pulverizing the above-mentioned raw materialpowder by means of a wet medium stirring mill to adjust the specificsurface area or the median diameter of particle size distribution, thestep of molding the pulverized raw material to form a molded product andthe step of sintering the molded product in an oxygen atmosphere at 1250to 1450° C.

The method (1), in which the specific surface area is adjusted, and themethod (2), in which the median diameter of particle size distributionis adjusted, will be described below.

(1) Production Method in which the Specific Surface Area is Adjusted

In this method, mixed powder containing the following powder (a) to (c)is used as the raw material powder.

(a) Indium oxide powder with a specific surface area of 6 to 10 m²/g(b) Gallium oxide powder with a specific surface area of 5 to 10 m²/g(c) Zinc oxide powder with a specific surface area of 2 to 4 m²/g

As mentioned later, in addition to the components (a) to (c), a fourthcomponent may be added. In this case, it is preferred that the totalamount of the three kinds of powder accounts for 90 wt % or more of theamount of entire raw material.

The specific surface area of the entire raw material mixed powder is 5to 8 m²/g.

By allowing the specific surface area of each oxide to be within theabove-mentioned range, mixing/pulverization can be conducted moreefficiently.

The specific surface area of each raw material powder is a valuemeasured by the BET method. The specific surface area can be adjusted bypulverizing the powder by a dry pulverizing method, a wet pulverizingmethod, or the like.

In the invention, it is preferred that the specific surface area ofindium oxide and the specific surface area of gallium oxide be almostthe same. This enables more efficient pulverization and mixing. It ispreferred that the difference in specific surface area among the rawmaterial powder be 3 m²/g or less. If the difference in specific surfacearea is large, effective pulverization and mixing may not be conducted,and gallium oxide powder may remain in the resulting sintered body.

The amount ratio of indium oxide and gallium oxide can be appropriatelyadjusted according to application or the like. In order to reduce thebulk resistance of the target and to conduct stable sputtering, theamount ratio (molar ratio) of indium oxide and gallium oxide is adjustedsuch that the amount of indium oxide and the amount of gallium oxidebecome the same or such that the amount of indium oxide is larger thanthe amount of gallium oxide. If the molar ratio of gallium oxide becomeslarger than the molar ratio of indium oxide, abnormal discharge may becaused since excessive gallium oxide particles may be present in thetarget.

The amount of zinc oxide is preferably the same as or smaller than thetotal amount (molar ratio) of indium oxide and gallium oxide.

Specifically, it is preferred that indium oxide, gallium oxide and zincoxide be weighed such that indium oxide:gallium oxide:zinc oxide (weightratio) be almost 45:30:25 (In:Ga:Zn=1:1:1, molar ratio) or 50:35:15(In₂O₃:Ga₂O₃:ZnO=1:1:1, molar ratio).

By mixing and pulverizing the above-mentioned raw material powder bymeans of a wet medium stirring mill, the specific surface area of theraw material powder is increased by 1.0 to 3.0 m²/g than the specificsurface area of the raw material powder before pulverization. Due tosuch adjustment, a high-density sintered body for an IGZO sputteringtarget can be obtained without the need of the prefiring step.

If an increase in specific surface area after pulverization is less than1.0 m²/g, the density of a sintered body after the sintering step is notincreased. If the specific surface area is increased by an amountexceeding 3.0 m²/g, the amount of admixed impurities (contaminants) froma pulverizer or the like at the time of pulverization is increased. Anincrease in specific surface area after pulverization is preferably 1.5to 2.5 m²/g.

Here, the specific surface area of the raw material mixed powder beforepulverization means the specific surface area measured in the statewhere each oxide powder has been mixed.

As the wet medium stirring mill, a commercially available apparatus,such as a beads mill, a ball mill, a roll mill, a planetary mill and ajet mill, can be used.

If a beads mill is used, as a pulverizing medium (beads), it ispreferable to use zirconia, alumina, quartz, titania silicon nitride,stainless steel, mullite, glass beads, SiC or the like, and the particlediameter thereof is preferably about 0.1 to 2 mm.

In order to allow the specific surface area of the mixed powder to beincreased by 1.0 to 3.0 m²/g than the specific surface area of the rawmaterial mixed powder before pulverization, the treatment time, the typeof beads, the particle diameter or the like may be appropriatelyadjusted. These conditions are required to be adjusted by means of anapparatus used.

The raw material after the above-mentioned pulverization is dried bymeans of a spray dryer or the like, followed by molding. The molding isperformed by a known method, such as pressure molding and cold isostaticpressing.

Then, the resulting molded product is sintered to obtain a sinteredbody.

The sintering temperature is controlled to 1250° C. to 1450° C.,preferably 1350° C. to 1450° C. By circulating oxygen or under an oxygenpressure, sintering is performed in an oxygen atmosphere. If thesintering temperature is lower than 1250° C., the density of thesintered body may not be increased. If the sintering temperature exceeds1450° C., the composition of the sintered body may vary or voids may begenerated in a sintered body due to evaporation. The sintering time is 2to 72 hours, preferably 20 to 48 hours.

Evaporation of zinc can be suppressed by conducting sintering in anatmosphere of oxygen, whereby a sintered body having no voids can beobtained. As a result, the density of a sintered body can be 6.0 g/cm³or more.

In addition, it is possible to obtain a sintered body having a bulkresistance of less than 5 mΩcm without the reduction step. If a sinteredbody has a bulk resistance of 5 mΩcm or more, abnormal discharge may beinduced or foreign matters (nodules) may be generated during sputtering.

(2) Production Method in which the Median Diameter is Adjusted

In this method, the mixed powder containing the following powder (a′) to(c′) is used.

(a′) Indium oxide powder with a median diameter of particle sizedistribution of 1 to 2 μm(b′) Gallium oxide powder with a median diameter of particle sizedistribution of 1 to 2 μm(c′) Zinc oxide powder with a median diameter of particle sizedistribution of 0.8 to 1.6 μm

In addition to the components (a) to (c), a fourth component may beadded. In this case, it is preferred that the total of the three kindsof powder accounts for 90 wt % or more of the entire raw material.

The median diameter of particle size distribution of the mixed powder asa raw material is 1.0 to 1.9 μm.

By allowing the specific surface area of each oxide to be within theabove-mentioned range, mixing/pulverization can be conducted moreefficiently.

Here, the median diameter of particle size distribution is a valuemeasured by means of a particle distribution meter. The median diametercan be adjusted by classification after performing dry pulverization andwet pulverization.

It is preferable to use powder in which the median diameter of indiumoxide and the median diameter of gallium oxide are almost the same. As aresult, mixing/pulverization can be performed more efficiency. It ispreferred that the difference in median diameter among the raw materialpowder be 1 μm or less. If the difference in median diameter is large,efficient mixing/pulverization may not be performed, and as a result,gallium oxide particles may remain in the resulting sintered body.

The amount ratio of indium oxide and gallium oxide, and thepulverization step are the same as mentioned in (1) above.

By the pulverization step, the median diameter after pulverizationbecomes 0.6 to 1 μm. It is preferred that the median diameter of the rawmaterial be changed before and after pulverization by 0.1 μm or more. Byusing the thus-prepared raw material powder, it is possible to obtain ahigh-density IGZO sputtering target without the need of the prefiringstep. If the median diameter after pulverization exceeds 1 μm, thedensity of a sintered body does not increase. If the median diameterafter pulverization is less than 0.6 μm, the amount of impurities from apulverizer or the like during pulverization increases.

Here, the median diameter after pulverization means the median diameterof the entire mixed powder.

The raw material after pulverization is molded and sintered to produce asintered body. The molding and sintering may be performed in the samemanner as in (1) mentioned above.

The sintered body prepared in (1) or (2) as mentioned above is thensubjected to polishing, cleaning or the like as in the above-mentionedfirst invention, whereby a sputtering target is obtained.

By conducting sputtering using a sputtering target after bonding, it ispossible to obtain an oxide semiconductor film containing oxides of In,Ga and Zn as main components. According to the production method of theinvention, not only the productivity of the sputtering target isimproved, but also the density of the resulting sputtering target can beas high as 6.0 g/cm³ or more. Therefore, an oxide semiconductor filmexcellent in film properties, suffering from only a small amount ofnodules or particles, can be obtained. Although depending on thecomposition, the upper limit of the density of the sputtering target isabout 6.8 g/cm³.

In the invention, in order to further decrease the bulk resistance ofthe sputtering target, a metal element with an atomic valency oftetravalency or higher may be contained in an amount of 200 to 5000 ppm(atomic ratio) in a sintered body. Specifically, in addition to indiumoxide, gallium oxide and zinc oxide, SnO₂, ZrO₂, CeO₂, GeO₂, TiO₂, HfO₂or the like may be added.

In the production method according to the invention, as far as indiumoxide, gallium oxide and zinc oxide are contained as main components,other components for improving the properties of the sputtering targetmay be contained in the raw material powder. For example, alanthanoid-based element with an atomic valency of positive trivalencymay be added, for example.

The resulting oxide semiconductor film is amorphous, and exhibits stablesemiconductor properties with no carrier generation effects (dopingeffects) even though a metal element with an atomic valency oftetravalency or higher is added.

EXAMPLES

The invention will be explained by way of examples while comparing themwith comparative examples. The following examples only show preferableexamples, and the invention are by no ways limited by these examples.Therefore, various modifications based on the technical concept of theinvention or other examples are included in the invention.

First Invention

The method for measuring the properties of the sputtering targetprepared in examples and comparative examples will be shown below.

(1) Density

Density was calculated from the weight and external dimension of atarget which has been cut out with a specific dimension.

(2) Bulk Resistance of Target

Bulk resistance was measured by the four probe method by means of aresistivity meter (Rhoresta, manufactured by Mitsubishi ChemicalCorporation).

(3) Structure of oxide present in target

The structure of an oxide was identified by analyzing a chart obtainedby the X-ray diffraction method.

(4) Specific Surface Area of Raw Material Powder

The specific surface area of the raw material powder was measured by theBET method.

(5) Median Diameter of Raw Material Powder

The median diameter of the raw material powder was measured by means ofa particle size distribution measuring apparatus.

(6) Average Transverse Rupture Strength

The average transverse rupture strength was measured by the three-pointbending test.

(7) Weibull Modulus of Sputtering Target

By the median rank method, the cumulative probability of failure againstthe bending strength and the Weibull plot in the single mode wereobtained, whereby a Weibull modulus (m value) showing the variation ofprobability of failure was obtained. The Weibull modulus (m value) wasobtained by using the linear regressive line.

Example 1

99.99%-pure indium oxide powder with a specific surface area of 6 m²/g,99.99%-pure gallium oxide powder with a specific surface area of 6 m²/g,and 99.99%-pure zinc oxide powder with a specific surface area of 3 m²/gwere weighed such that the weight ratio of In₂O₃:Ga₂O₃:ZnO became45:30:25. The powder was then subjected to mixing/pulverization by meansof a wet medium stirring mill. As the medium of the wet medium stirringmill, zirconia beads with a diameter of 1 mm were used.

After the mixing/pulverization, the specific surface area of each rawmaterial powder was increased by 2 m²/g. The raw material powder wasthen dried by means of a spray dryer. The resulting mixed powder wasplaced in a mold, and then subject to pressure molding by means of acold pressing machine, whereby a molded product was produced.

The resulting molded product was sintered for 4 hours at a hightemperature of 1400° C. in an oxygen atmosphere by circulating oxygen.As a result, a sintered body for an IGZO sputtering target (sputteringtarget) having a density of 6.06 g/cm³ was obtained without conductingthe prefiring step. The X-ray diffraction analysis confirmed thepresence of crystals of ZnGa₂O₄ and InGaZna₄ in the sintered body. FIG.1 shows an X-ray diffraction analysis chart.

The sintered body had a bulk resistance of 4.2 mΩcm.

The amount of impurities in this sintered body was measured by the ICPanalysis. The results showed that the content of each of Fe, Al, Si, Niand Cu was less than 10 ppm.

Example 2

Indium oxide powder with a median diameter of 1.5 μm, gallium oxidepowder with a median diameter of 2.0 μm, and zinc oxide powder with amedian diameter of 1.0 μm were weighed such that the weight ratio ofIn₂O₃:Ga₂O₃:ZnO became almost 55:25:20. The powder was then subjected tomixing/pulverization by means of a wet medium stirring mill. As themedium of the wet medium stirring mill, zirconia beads with a diameterof 1 mm were used.

After the mixing/pulverization, the average median diameter of each rawmaterial was allowed to be 0.8 μm. The raw material powder was thendried by means of a spray dryer. The resulting mixed powder was placedin a mold, and then subject to pressure molding by means of a coldpressing machine, whereby a molded product was produced.

The resulting molded product was sintered for 4 hours at a hightemperature of 1400° C. in an oxygen atmosphere by circulating oxygen.As a result, a sintered body for an IGZO sputtering target having adensity of 6.14 g/cm³ was obtained without conducting the prefiringstep. The X-ray diffraction analysis confirmed the presence of crystalsof ZnGa₂O₄, InGaZna₄ and In₂O₃(ZnO)₄ in the sintered body. FIG. 2 showsthe X-ray diffraction analysis chart.

The sintered body had a bulk resistance of 3.8 mΩcm.

Example 3

Indium oxide powder with a median diameter of 1.5 μm, gallium oxidepowder with a median diameter of 2.0 μm, and zinc oxide powder with amedian diameter of 1.0 μm were weighed such that the weight ratio ofIn₂O₃:Ga₂O₃:ZnO became almost 35:25:40. The powder was then subjected tomixing/pulverization by means of a wet medium stirring mill. As themedium of the wet medium stirring mill, zirconia beads with a diameterof 1 mm were used.

After the mixing/pulverization, the average median diameter of each rawmaterial was 0.8 μm. The raw material powder was then dried by means ofa spray dryer. The resulting mixed powder was placed in a mold, and thensubject to pressure molding by means of a cold pressing machine, wherebya molded product was produced.

The resulting molded product was sintered for 4 hours at a hightemperature of 1400° C. in an oxygen atmosphere by circulating oxygen.As a result, a sintered body for an IGZO sputtering target having adensity of 6.02 g/cm³ was obtained without conducting the prefiringstep. The X-ray diffraction analysis confirmed that crystals of ZnGa₂O₄and InGaZnO₄ were present in the sintered body. FIG. 3 shows an X-raydiffraction analysis chart.

The sintered body had a bulk resistance of 4.9 mΩcm.

Comparative Example 1

Indium oxide powder with a median diameter of 1.5 μm, gallium oxidepowder with a median diameter of 2.0 μm, and zinc oxide powder with amedian diameter of 1.0 μm were weighed such that the weight ratio ofIn₂O₃:Ga₂O₃:ZnO became almost 34:46:20. The powder was then subjected tomixing/pulverization by means of a wet medium stirring mill. As themedium of the wet medium stirring mill, zirconia beads with a diameterof 1 mm were used.

After the mixing/pulverization, the average median diameter of each rawmaterial was allowed to be 0.8 μm. The raw material powder was thendried by means of a spray dryer. The resulting mixed powder was placedin a mold, and then subject to pressure molding by means of a coldpressing machine, whereby a molded product was produced.

The resulting molded product was sintered for 4 hours at a hightemperature of 1200° C. in an oxygen atmosphere by circulating oxygen.As a result, a sintered body for an IGZO sputtering target having adensity of 5.85 g/cm³ was obtained without conducting the prefiringstep. As a result of the X-ray diffraction analysis, it was confirmedthat, although crystals of ZnGa₂O₄ were present in the sintered body,InGaO₃(ZnO)_(m) was not generated. FIG. 4 shows an X-ray diffractionanalysis chart.

The sintered body had a bulk resistance of 450 mΩcm.

Examples 4 and 5

Then, the sintered body for an IGZO sputtering target produced inExample 1 was subjected to fine polishing (Example 4: fine polishing,Example 5: surface grinding in the longitudinal direction), whereby asputtering target was produced. As for the structure of thethus-produced sputtering target, the target surface was analyzed byobserving a secondary electron image of a scanning electron microscope(SEM). As a result, the average particle diameters of ZnGa₂O₄ crystalsin the targets in Examples 4 and 5 were found to be 4.4 μm. The surfaceroughness was measured by means of a surface roughness meter. As aresult, it was found that the surface roughness Ra of the target inExample 4 was 0.5 μm and the surface roughness Ra of the target inExample 5 was 1.8 μm.

Comparative Example 2

The sintered body for an IGZO sputtering target produced in ComparativeExample 1 was subjected to fine polishing (surface grinding in thelongitudinal direction), whereby a sputtering target was produced. Asfor the structure of the thus produced sputtering target, the targetsurface was analyzed by observing a secondary electron image of ascanning electron microscope (SEM). The average particle diameter ofZnGa₂O₄ crystals in the target was 14 μm. The surface roughness wasmeasured by means of a surface roughness meter. As a result, it wasfound that the surface roughness Ra of the target was 3.5 μm.

Then, as for each of the sputtering targets obtained in Examples 4 and5, and Comparative Example 2, the Weibull modulus and the averagetransverse rupture strength were evaluated. The results are shown inTable 1.

TABLE 1 Average Surface transverse roughness Weibull rupture Ra modulusstrength Grinding condition (μm) [—] [MPa] Example 4 Polishing 0.5 10.458 Example 5 Surface grinding 1.8 10.2 54 in the longitudinal directionComparative Surface grinding 3.5 9.1 46 Example 2 in the longitudinaldirection

A larger Weibull modulus value means that no variation can be observedin the maximum value of the non-destructive stress. From the resultsshown in Table 1, it was confirmed that the sputtering target of theinvention was a stable target suffering from only a small degree ofvariation.

The surface roughness after surface grinding normally corresponds to theparticle diameter of the crystals. If the particle diameter is notuniform, Ra tends to increase. The transverse rupture strength islowered with an increase in Ra.

From the results shown in Table 1, it was confirmed that the sputteringtarget of the invention has a high quality since it has a small crystalparticle diameter and a small surface roughness.

Example 6

The target produced in Example 4 (diameter: 4 inches, thickness: 5 mm)was bonded to a backing plate, and then installed in a DC sputteringfilm formation apparatus. Under an argon atmosphere of 0.3 Pa,continuous sputtering was conducted at 100 W for 100 hours. Nodulesgenerated on the target surface were observed. As a result of theobservation, no nodules were generated on the surface.

Comparative Example 3

The target produced in Comparative Example 2 (diameter: 4 inches,thickness: 5 mm) was bonded to a backing plate, and then installed in aDC sputtering film formation apparatus. Under an argon atmosphere of 0.3Pa, continuous sputtering was conducted at 100 W for 100 hours. Nodulesgenerated on the target surface were observed. It was found that noduleswere generated on almost half of the target surface.

Example 7

A sintered body was prepared in the same manner as in Example 1, exceptthat tin oxide, zirconium oxide, germanium oxide, cerium oxide, niobiumoxide, tantalum oxide, molybdenum oxide or tungsten oxide (an oxide of ametal element with an atomic valency of positive tetravalency or higher)was added. The bulk resistance of the sintered body was measured. Therelationship between the amount of a metal element with an atomicvalency of positive tetravalency or higher and the bulk resistance ofthe sintered body is shown in FIG. 5. A sintered body for an IGZOsputtering target was obtained by using tin as the metal element with anatomic valency of positive tetravalency or higher and by adding tin suchthat [tin element/total metal elements:atomic ratio] became 0.001. TheX-ray diffraction chart of the resulting sintered body for an IGZOsputtering target is shown in FIG. 6.

As is apparent from FIG. 5, by adding a metal element with an atomicvalency of positive tetravalency or higher, the bulk resistance waslowered.

Second Invention Example 8

Indium oxide powder with a specific surface area of 6 m²/g, galliumoxide powder with a specific surface area of 6 m²/g, and zinc oxidepowder with a specific surface area of 3 m²/g were weighed such that theweight ratio became 45:30:25. Furthermore, as the metal element with anatomic valency of positive tetravalency or higher, SnO₂ was added suchthat the content of an Sn element relative to the total metal elements[Sn/(In+Ga+Zn+Sn):weight ratio] became 600 ppm.

The raw material mixed powder was subjected to mixing/pulverization bymeans of a wet medium stirring mill. As the medium, zirconia beads witha diameter of 1 mm were used. After the mixing/pulverization, thespecific surface area of the raw material powder was increased by 2m²/g. The raw material powder was then dried by means of a spray dryer.

The resulting mixed powder was placed in a mold, and then subject topressure molding by means of a cold pressing machine, whereby a moldedproduct was produced. The resulting molded product was sintered at 1550°C. for 8 hours in an oxygen atmosphere by circulating oxygen. As aresult, a sintered body for an IGZO sputtering target having a densityof 6.12 g/cm³ was obtained without conducting the prefiring step.

The density of the sintered body was calculated from the weight and theexternal dimension of a sintered body which had been cut out into aspecific size.

The sintered body was analyzed by the X-ray diffraction method. FIG. 7is an X-ray diffraction chart of the sintered body. From FIG. 7,presence of crystals of InGaZnO₄ could be confirmed. Since a peakderived from a metal oxide other than InGaZnO₄ could not be observed, itwas confirmed that a sintered body containing InGaZnO₄ as maincomponents was obtained.

The bulk resistance of the thus obtained sintered body was measured bythe four probe method by means of a resistivity meter (Rhoresta,manufactured by Mitsubishi Chemical Corporation). It was found that thebulk resistance was 0.95×10⁻³ Ωcm.

Comparative Example 4

A sintered body was produced in the same manner as in Example 8, exceptthat a metal oxide which contained a metal element with an atomicvalency of positive tetravalency or higher (tin oxide) was not added.

The density of this sintered body was found to be 5.98 g/cm³. As aresult of the X-ray diffraction analysis, it was confirmed that asintered body containing InGaZnO₄ as a main component was obtained,since crystals of InGaZnO₄ were present and a peak derived from a metaloxide other than InGaZnO₄ was not observed.

The bulk resistance of this sintered body was 0.018 Ωcm.

FIG. 8 is a graph showing the relationship between the amount of a tinelement and the bulk resistance of the sintered body. FIG. 8 shows thebulk resistances of sintered bodies prepared in the same manner as inExample 8 except that the amount of the tin oxide powder was varied to500 ppm, 800 ppm and 1000 ppm and the bulk resistance of a sintered bodyprepared in Comparative Example 4 (the amount of a tin element was 0ppm). As is apparent from FIG. 8, by adding a tin element which is ametal element with an atomic valency of tetravalency or higher, the bulkresistance of a sintered body could be reduced.

Examples 9 to 15

Sintered bodies were prepared in the same manner as in Example 8, exceptthat, as the metal oxide containing a metal element with an atomicvalency of tetravalency or higher, a prescribed amount of an oxide shownin Table 2 was used instead of tin oxide. The bulk resistances of thesintered body are shown in Table 2.

TABLE 2 Amount added Bulk resistance (Ω Metal oxide (weight ppm) cm)Example 9 Zirconium 1010 0.94 × 10⁻³ oxide Example 10 Germanium 10200.89 × 10⁻³ oxide Example 11 Cerium 2000 0.92 × 10⁻³ oxide Example 12Niobium 5000 0.95 × 10⁻³ oxide Example 13 Tantalum 10000 0.96 × 10⁻³oxide Example 14 Molybdenum 1500 0.92 × 10⁻³ oxide Example 15 Tungsten1020 0.91 × 10⁻³ oxide

FIG. 9 shows the results of Examples 9 to 15, specifically, it is agraph showing the relationship between the amount of the metal elementwith an atomic valency of positive tetravalency or higher and the bulkresistance of the sintered body. As is apparent from FIG. 9, the bulkresistance is lowered by adding a metal element with an atomic valencyof positive tetravalency or higher.

Third Invention Example 16

The following oxide powder was used and weighed as the raw materialmixed powder. The specific surface area was measured by the BET method.

(a) Indium oxide powder: 45 wt %, specific surface area: 6 m²/g(b) Gallium oxide powder: 30 wt %, specific surface area: 6 m²/g(c) Zinc oxide powder: 25 wt %, specific surface area: 3 m²/g

The specific surface area of the entire mix powder comprising (a) to (c)was 5.3 m²/g.

The above-mentioned mixed powder was subjected to mixing/pulverizationby means of a wet medium stirring mill. As the medium of the wet mediumstirring mill, zirconia beads with a diameter of 1 mm were used. Duringthe pulverization, the specific surface area of the mixed powder waschecked, thereby to allow the specific surface area to increase by 2m²/g.

The raw material powder was then dried by means of a spray dryer. Theresulting mixed powder was placed in a mold (diameter: 150 mm,thickness: 20 mm), and then subject to pressure molding by means of acold pressing machine.

After the molding, the resulting molded product was sintered for 40hours at 1400° C. in an oxygen atmosphere by circulating oxygen.

The density of the thus produced sintered body was calculated from theweight and external dimension of a piece of the sintered body which hadbeen cut out into a predetermined size. The results showed that thedensity of the sintered body was 6.15 g/cm³. As mentioned hereinabove, ahigh-density sintered body for an IGZO sputtering target could beobtained without conducting the prefiring step.

As a result of the analysis on the chart obtained by the X-raydiffraction method, it was confirmed that crystals of InGaZna₄ andcrystals of Ga₂ZnO₄ were present in the sintered body.

The bulk resistance of the sintered body was measured by means of aresistivity meter (Rhoresta, manufactured by Mitsubishi ChemicalCorporation). It was found that the sintered body had a bulk resistanceof 4.2 mΩcm.

Example 17

As the raw material mixed powder, the following oxide powder was usedand weighed. The median diameter was measured by means of a particlesize distribution meter.

(a′) Indium oxide powder: 50 wt %, median diameter: 1.5 μm(b′) Gallium oxide powder: 35 wt %, median diameter: 2.0 μm(c′) Zinc oxide powder: 15 wt %, median diameter: 1.0 μm

The median diameter of the entire mixed powder comprising (a′) to (c′)was 1.6 μm.

In the same manner as in Example 16, the above-mentioned mixed powderwas subjected to mixing/pulverization by means of a wet medium stirringmill. During the pulverization, the median diameter of the mixed powderwas checked, thereby to allow the median diameter to be 0.9 μm.

In the same manner as in Example 16, the mixed powder was molded,followed by sintering to produce a sintered body, and the resultingsintered body was evaluated.

The results showed that the density of the sintered body was 6.05 g/cm³.A high-density sintered body for an IGZO sputtering target could beobtained without conducting the prefiring step.

It was confirmed that crystals of InGaZnO₄ and crystals of Ga₂ZnO₄ werepresent in the sintered body.

The bulk resistance of the sintered body was 3.8 mΩcm.

Comparative Example 5

The following oxide powder was used and weighed as the raw materialmixed powder.

(a) Indium oxide powder: 45 wt %, specific surface area: 9 m²/g(b) Gallium oxide powder: 30 wt %, specific surface area: 4 m²/g(c) Zinc oxide powder: 25 wt %, specific surface area: 3 m²/g

The specific surface area of the entire mixed powder comprising (a) to(c) was 6 m²/g.

The raw material mixed powder was subjected to mixing/pulverization bymeans of a wet medium stirring mill in the same manner as in Example 16.During pulverization, the specific surface area of the mixed powder waschecked, thereby to allow the specific area of the mixed powder to beincreased by 1.4 m²/g.

Then, molding of the mixed powder and sintering were conducted in thesame manner as in Example 16, except that the sintering condition waschanged to 40 hours at 1400° C. under atmospheric conditions.

The density of the resulting sintered body was 5.76 g/cm³. That is, inthis comparative example, a sintered body having a low density wasobtained.

In addition, the bulk resistance of the sintered body was 140 mΩcm dueto the absence of the reduction step.

In the sintered body, crystals which appeared to be gallium oxide werepresent.

Comparative Example 6

As the raw material mixed powder, the following oxide powder was usedand weighed.

(a′) Indium oxide powder: 50 wt %, median diameter: 2.5 μm(b′) Gallium oxide powder: 35 wt %, median diameter: 2.5 μm(c′) Zinc oxide powder: 15 wt %, median diameter: 2.0 μm

The median diameter of the entire mixed powder comprising (a′) to (c′)was 2.4 μm.

In the same manner as in Example 16, the above-mentioned mixed powderwas subjected to mixing/pulverization by means of a wet medium stirringmill. During the pulverization, the median diameter of the mixed powderwas checked, there by to allow the median diameter to be 2.1 μm.

In the same manner as in Example 16, the mixed powder was molded,following by sintering to produce a sintered body, and the resultingsintered body was evaluated, except that the sintering condition waschanged to 10 hours at 1400° C. under atmospheric conditions.

The density of the resulting sintered body was 5.85 g/cm³. That is, inthis comparative example, a sintered body having a low density wasobtained.

In addition, the bulk resistance of the sintered body was 160 mΩcm dueto the absence of the reduction step.

In the sintered body, crystals which appeared to be gallium oxide werepresent.

Comparative Example 7

A prefiring step was conducted in Comparative Example 5. Specifically,the same mixed powder as in Comparative Example 5 was prefired at 1200°C. for 10 hours. The powder after the prefiring had a specific surfacearea of 2 m²/g.

The thus prefired powder was pulverized by means of a wet mediumstirring mill, whereby the specific surface area thereof was increasedby 2 m²/g. Then, the mixed powder was subjected to drying and pressuremolding in the same manner as in Example 16. Thereafter, the moldedproduct was sintered at 1450° C. for 4 hours in an oxygen atmosphere,whereby a sintered body was produced.

The sintered body had a density of 5.83 g/cm³. As compared withComparative Example 5, the density could be increased. However, ascompared with Examples 16 and 17, in which the prefiring step was notconducted, poorer results were obtained. In addition, provision of theprefiring step impaired the productivity of a sintered body.

This sintered body was subjected to a reduction treatment at 500° C. for5 hours in a nitrogen stream. As a result, the bulk density of thesintered body was 23 mΩcm.

Comparative Example 8

The same mixed powder as in Comparative Example 6 was prefired at 1200°C. for 10 hours. The specific surface area of the prefired powder was 2m²/g.

The prefired powder was pulverized by means of a wet medium stirringmill to increase the specific surface area by 2 m²/g. Then, the mixedpowder was subjected to drying and pressure molding in the same manneras in Example 16. Thereafter, the molded product was sintered at 1450°C. for 40 hours in an oxygen atmosphere, whereby a sintered body wasproduced.

The sintered body had a density of 5.94 g/cm³. As compared withComparative Example 6, the density could be increased. However, ascompared with Examples 16 and 17, in which the prefiring step was notconducted, poorer results were obtained. In addition, provision of theprefiring step impaired the productivity of a sintered body.

This sintered body was subjected to a reduction treatment at 500° C. for5 hours in a nitrogen stream. The bulk density of the sintered body was23 mΩcm.

INDUSTRIAL APPLICABILITY

The target of the invention is suitable as a target to obtain, by thesputtering method, a transparent conductive film and an oxidesemiconductor film for various applications including a transparentconductive film for a liquid crystal display (LCD) apparatus, atransparent conductive film for an electroluminescence (EL) displayapparatus and a transparent conductive film for a solar cell. Forexample, it is possible to obtain a transparent conductive film for anelectrode of an organic EL device, a transparent conductive film for asemi-transmitting/semi-reflecting LCD, and oxide semiconductor film fordriving a liquid crystal display and an oxide semiconductor film fordriving an organic EL device. Furthermore, it is suitable as a rawmaterial of an oxide semiconductor film for a switching device, adriving circuit device or the like of a liquid display apparatus, a thinfilm electroluminescence display apparatus, an electrophoresis displaydevice, a moving particle display device or the like.

The method for producing a sputtering target of the invention is anexcellent production method which can improve the productivity of atarget, since the prefiring step or the reduction step is not necessary.

1-13. (canceled)
 14. A sputtering target comprising a compound shown byInGaZnO₄ as a main component, which further contains a metal elementwith an atomic valency of positive tetravalency or higher wherein thecontent of the metal element with an atomic valency of positivetetravalency or higher is 100 ppm to 10000 ppm relative to the totalmetal elements in the sputtering target.
 15. (canceled)
 16. Thesputtering target according to claim 14, wherein the content of themetal element with an atomic valency of positive tetravalency or higheris 200 ppm to 5000 ppm relative to the total metal elements in thesputtering target.
 17. The sputtering target according to claim 14,wherein the content of the metal element with an atomic valency ofpositive tetravalency or higher is 500 ppm to 2000 ppm relative to thetotal metal elements in the sputtering target.
 18. The sputtering targetaccording to claim 14, which has a bulk resistance of less than 1×10⁻³Ωcm.
 19. The sputtering target according to claim 14, wherein the metalelement with an atomic valency of positive tetravalency or higher is atleast one element selected from the group consisting of tin, zirconium,germanium, cerium, niobium, tantalum, molybdenum and tungsten. 20-23.(canceled)